Despite extensive pre-clinical evaluation, the cardiac dysfunction associated with the Herceptin erbB2/HER2/neu receptor antibody therapy was discovered only late in breast cancer clinical trials. The adverse cardiac effect of Herceptin therapy has taught the biopharmaceutical industry and the medical community two important lessons. First, the toxic side effects of highly-specific drugs (especially antibodies) targeted against human cellular proteins may occur exclusively in humans. Potential organ toxicity of these agents must therefore be tested in human tissues. Secondly, the cardiac failure associated with Herceptin has demonstrated that signaling molecules that drive cell proliferation remain critically important in maintaining normal cellular function even in terminally-differentiated cardiomyocytes. Thus, as "designer" drug therapy and cancer vaccine development move toward specifically targeting cell signaling molecules involved in cell growth, there is a heightened urgency to develop models to better predict the cardiac safety of these agents. With this intention, we have recently developed a system for culturing human myocardium on a cushion of feeder cells from the developing rat heart. In co-culture, explanted human myocardium remains alive, healthy and spontaneously beating for several months. We have successfully used this system, which takes advantage of a small fragment of cardiac tissue that is otherwise discarded during routine cardiac surgery, to study the problem of Herceptin cardiomyopathy. In the R21 phase of this grant application, we propose to define the precise growth parameters of co- cultured human myocardial tissue, then develop a reliable and reproducible quantitative biochemical marker assay for drug-induced injury of human myocardium. This assay will be tested and validated using anthracyclines, well known cardiotoxic drugs. In the R33 phase of this grant application, we propose to use cultured human myocardium to evaluate the cardiac injury potential of all other newly emerging anticancer drugs that specifically interfere with cell signaling pathways. In addition to the culture system, we propose to explore in the R21 phase the possibility of growing human myocardial tissue as a xenograft in nude mice, where vascularization should ensure the cardiac tissue's long-term viability. We hypothesize that human myocardial tissue xenografts may be particularly useful for study of the potential cardiotoxicity of anticancer drugs targeted at matrix metalloproteinases and angiogenesis factors, and in the R33 phase intend to develop an assay for human myocardial tissue remodeling, angiogenesis and ischemia in nude mice. The results of the proposed studies will have important implications for improving the cardiac drug discovery and testing programs.